Cladding mode light removal structure and laser apparatus

Abstract

A cladding mode light removal structure includes: an input-side exposure portion where a covering is removed from an end of an input-side optical fiber to expose a cladding of the input-side optical fiber; an output-side exposure portion where a covering is removed from an end of an output-side optical fiber to expose an output-side cladding of the output-side optical fiber; a fusion splice portion at which the input-side exposure portion and the output-side exposure portion are connected by fusion splice; a fiber housing portion having an accommodation space that receives the input-side exposure portion, the output-side exposure portion, and the fusion splice portion; a high refractive index resin having a refractive index equal to or higher than that of the cladding exposed at the input-side exposure portion; and a low refractive index portion of a medium.

The invention claimed is: 
     
       1. A cladding mode light removal structure for removing cladding mode light that propagates through an optical fiber cladding, the cladding mode light removal structure comprising:
 an input-side cladding exposure portion where a covering is removed from an end of an input-side optical fiber and an input-side cladding of the input-side optical fiber is exposed; 
 an output-side cladding exposure portion where a covering is removed from an end of an output-side optical fiber and an output-side cladding of the output-side optical fiber is exposed; 
 a fusion splice portion at which the input-side cladding exposure portion and the output-side cladding exposure portion are connected to each other by fusion splice; 
 a fiber housing portion having an accommodation space that receives the input-side cladding exposure portion, the output-side cladding exposure portion, and the fusion splice portion; 
 a high refractive index resin having a refractive index that is equal to or higher than a refractive index of the input-side cladding exposed at the input-side cladding exposure portion, wherein
 the high refractive index resin is disposed within the accommodation space of the fiber housing portion and surrounds a downstream end of the covering of the input-side optical fiber, an entire portion of the input-side cladding exposed at the input-side cladding exposure portion, the fusion splice portion, and a portion of the output-side cladding exposed at the output-side cladding exposure portion; and 
 
 a low refractive index portion of a medium having a refractive index lower than a refractive index of the output-side cladding of the output-side cladding exposure portion, wherein
 the low refractive index portion is disposed around the output-side cladding exposure portion between the high refractive index resin and the covering of the output-side optical fiber along an axial direction of the output-side optical fiber. 
 
 
     
     
       2. The cladding mode light removal structure as recited in  claim 1 , wherein a length x of the low refractive index portion along the axial direction of the output-side optical fiber satisfies 
       
         
           
             
               x 
               > 
               
                 
                   
                     
                       D 
                       1 
                     
                     - 
                     
                       D 
                       2 
                     
                   
                   2 
                 
                 
                   tan 
                   ⁡ 
                   
                     ( 
                     
                       
                         sin 
                         
                           - 
                           1 
                         
                       
                       ⁡ 
                       
                         ( 
                         
                           
                             
                               n 
                               H 
                             
                             
                               n 
                               L 
                             
                           
                           × 
                           NA 
                         
                         ) 
                       
                     
                     ) 
                   
                 
               
             
           
         
         where D 1  is a diameter of the covering of the output-side optical fiber, 
         D 2  is a diameter of the output-side cladding of the output-side optical fiber, 
         n H  is a refractive index of the high refractive index resin, 
         n L  is a refractive index of the medium of the low refractive index portion, and 
         NA is a numerical aperture of cladding mode light in the high refractive index resin. 
       
     
     
       3. The cladding mode light removal structure as recited in  claim 1 , wherein a diameter of the input-side cladding is greater than a diameter of the output-side cladding. 
     
     
       4. The cladding mode light removal structure as recited in  claim 1 , wherein the medium of the low refractive index portion comprises air. 
     
     
       5. A laser apparatus comprising:
 an optical fiber amplifier comprising:
 a pumping light source that outputs pumping light; and 
 an amplification optical fiber including a core into which a rare earth element to be excited by the pumping light from the pumping light source has been doped; and 
 
 the cladding mode light removal structure as recited in  claim 1 , wherein the input-side optical fiber corresponds to the amplification optical fiber of the optical fiber amplifier or an optical fiber located on a downstream side of the amplification optical fiber. 
 
     
     
       6. The laser apparatus as recited in  claim 5 , wherein a length x of the low refractive index portion along the axial direction of the output-side optical fiber satisfies 
       
         
           
             
               x 
               > 
               
                 
                   
                     
                       D 
                       1 
                     
                     - 
                     
                       D 
                       2 
                     
                   
                   2 
                 
                 
                   tan 
                   ⁡ 
                   
                     ( 
                     
                       
                         sin 
                         
                           - 
                           1 
                         
                       
                       ⁡ 
                       
                         ( 
                         
                           
                             
                               n 
                               H 
                             
                             
                               n 
                               L 
                             
                           
                           × 
                           NA 
                         
                         ) 
                       
                     
                     ) 
                   
                 
               
             
           
         
         where D 1  is a diameter of the covering of the output-side optical fiber, 
         D 2  is a diameter of the output-side cladding of the output-side optical fiber, 
         n H  is a refractive index of the high refractive index resin, 
         n L  is a refractive index of the medium of the low refractive index portion, and 
         NA is a numerical aperture of cladding mode light in the high refractive index resin. 
       
     
     
       7. The laser apparatus as recited in  claim 5 , wherein a diameter of the input-side cladding is greater than a diameter of the output-side cladding. 
     
     
       8. The laser apparatus as recited in  claim 5 , wherein the medium of the low refractive index portion comprises air.